The measurement of intracranial pressure (ICP) plays a critical role in several neurosurgical conditions. Various pathological processes such as hydrocephalus, tumors, and trauma can cause alterations in the pressure within the skull. If not adequately controlled, increases in intracranial pressure (due to accumulation of cerebrospinal fluid, blood clots, tumors, or brain swelling) can cause secondary damage to otherwise healthy brain tissue.
A number of technologies currently exist to monitor brain pressure. Many of these rely on invasive techniques with percutaneously implanted sensors. Wires or fiber optic cables are often used to transduce pressure information from electromechanical or optomechanical transducers, which relegates these technologies to short term use. At the end of use these sensors are withdrawn from the body. Several disadvantages are associated with such devices: 1) the presence of a percutaneous probe increases the chance of iatrogenic infections such as meningitis and cerebritis; 2) the probe must be withdrawn at the end of use, and so it is not reusable for subsequent episodes of suspected intracranial hypertension such as with hydrocephalus; and 3) the percutaneous cable is subject to mechanical failure and to inadvertent pull-out during routine patient care.
In an attempt to mitigate these disadvantages, numerous investigators have tried to develop non-invasive techniques for monitoring intracranial pressure. Such methods have employed mathematical correlations between physiological variables which can be transduced extracorporally such as blood pressure, heart rate, Doppler ultrasound of cerebral blood vessels, near-infrared (NIR) spectroscopy of cerebral oxygenation, retinal imaging, etc. While some success has been achieved in monitoring trends in ICP, no method has been fully successful in deriving the absolute intracranial pressure, and these known techniques have not gained significant clinical utility for monitoring ICP.
In another aspect of the present invention, a passive device based on two quiescent resonant tuned circuits is positioned subcutaneously on the patient's skull. While the concept of using a passive device for intracranial pressure monitoring exists in the prior art via Seylar, U.S. Pat. No. 4,114,606, a number of novel improvements are brought to bear in the present invention. In one such improvement, the device compensates for temperature, aging and stray capacitance by using two collocated and implanted circuits, the first circuit in contact with the intracranial space and thus experiencing the intracranial pressure; the second circuit sealed at a fixed and predetermined pressure. In another such improvement, the present invention incorporates a radio frequency identification (RF-ID) tag for holding calibration data and other critical data such as patient information and insertion date.
The presently described invention utilizes near-infrared beams to traverse biological tissue for the digital transmission of data.
Physiological parameters such as tissue oxygenation may be measured by comparing the absorption of specific optical wavelengths by the hemoglobin and cytochrome chromophores. This technology is omnipresent in the hospital setting in the form of pulse oximetry. In what is best disclosed as spectrophotometry, these aforementioned measurement techniques utilize analog means to derive quantitative measures of some physiological parameter via absorption of selected spectra. In a dramatic paradigm shift, the presently described invention utilizes an infrared beam to traverse biological tissue for the digital transmission of data.
The suitability of transmission of data across biological tissues via infrared beam is dependent primarily upon the attenuation of the light beam. From the modified Beer-Lambert equation, the attenuation, expressed in optical density, is:Attenuation(OD)=−log(I/Io)=Bμadp+G  (1)
Where “I” represents the transmitted light intensity, “Io” represents the incident intensity, “B” is a path length factor dependent upon the absorption coefficient “μa” and scattering coefficient “μs” “dp” represents the interoptode distance, and G represents a geometry-dependent factor.
The Near Infrared (NIR) spectrum is generally referred to as the frequency range from 750 to 2500 nm. In vivo measurements of NIR absorption during transillumination of the newborn infant brain suggest an optical density of 10 over interoptode distances of 8-9 cm. See Cope, M and Delpy, D. T. “System for long term measurement of cerebral blood and tissue oxygenation on newborn infants by near infrared transillumination.” Medicine, Biology, Engineering and Computing, 26(3):289-94, 1988. Assuming the light source and detector are collinear and antiparallel, the geometry-dependent factor, G, becomes negligible. Because biological tissue is an effective multiple scatterer of light, the effective path length traveled by a given photon can only be estimated. In a study measuring the water absorption peak at 975 nm and assuming average tissue water content, the path length of brain tissue is estimated at 4.3 times the interoptode distance. See Wray, S., Cope, M., Delpy, D. T., Wyatt, J. S. and Reynolds, E. O. R. “Characterization of the near infrared absorption spectra of cytochrome aa3 and hemoglobin for the non-invasive monitoring of cerebral oxygenation.” Biochimica Biophysica Acta 933:184-92, 1988. Thus, from the Beer-Lambert equations, the calculated absorption coefficient for human brain is approximately 0.26 cm2 with an assumed path length of 4.3. This is within the range of absorption coefficients (0.0434-0.456 cm2) quoted in the literature. See Svaasand, L. O. and Ellingsen, R. “Optical properties of brain.” Photochemistry and Photobiology, 38(3):293-9, 1983. In the studies of Tamura and Tamura, extracranial structures such as skin, muscle and bone had minimal effects on the NIR transmission-mode absorbance, presumably because the blood flow and oxygen consumption of these structures is low compared to that of cerebral cortex. See Tamura, M. and Tamura, T. “Non-invasive monitoring of brain oxygen sufficiency on cardiopulmonary bypass patients by near-infra-red laser spectrophotometry.” Medical and Biological Engineering and Computing 32:S151-6. The relatively minor contribution of scalp tissue to NIR absorption is further corroborated by Owen-Reece, Owen-Reece, H., Elwell, C. E., Wyatt, J. S. and Delpy, D. T., “The effect of scalp ischaemia on measurement of cerebral blood volume by near-infrared spectroscopy.” Physiological Measurements, November, 17(4):279-86, 1996. Thus, it is reasonable to expect that for a typical scalp thickness of 1 cm, the absorption would be somewhat less than: Bμadp=(4.3)(0.26)(1)=1.1, assuming that the geometry factor is negligible. Therefore, with an attenuation of one to two orders of magnitude and an NIR emitter output power of 5 mW, the transmitted light intensity is well within the sensitivity range of common silicon photodiodes.
Delpy, et al have investigated the relationship between attenuation and the transit time of light through tissue in an attempt to determine optical path length. See Delpy, D. T., Cope, M., van der Zee, P., Arridge, S., Wray, S. and Wyatt, J. “Estimation of optical path length through tissue from direct time of flight measurement.” Physics, Medicine and Biology 33(12): 1433-42, 1988. Temporal dispersion resulting from spatial and temporal delta functions of the input beam as it passes through scattering tissue may be described by the temporal spread point function (TSPF). Using a Monte Carlo model of light transport in tissue and experimentally derived (in vitro rat brain) scattering phase function at 783 nm, they computed the TSPF for a beam of light passing through a 1 cm thick slab of brain tissue. Estimates of path length based upon the time-of-flight of photons using the TSPF integrated over the exit surface, at all exits angles, and assuming radial symmetry, yields an average path length of 5.3 times the interoptode distance. The final photons to emerge from the tissue are calculated to have traveled 9.2 times the interoptode distance. The temporal dispersion of the light will limit the maximum transmission bandwidth:Fmax=1/t=c/dn  (2)
Where Fmax is the maximum transmission frequency, t is the time for the light to traverse the tissue, c is the speed of light, d is the distance traveled, and n is the refractive index.